Docs: Add missing 'w' (week) unit to SAMPLE BY reference (Hacktoberfest) (#252)

Fixes #57
What changed: I located the SQL reference documentation for SAMPLE BY
and added the missing time unit 'w' (week) to the units table in
sample-by.md.
Value: This corrects a factual inaccuracy and provides immediate value
to users needing weekly aggregation.

Co-authored-by: javier ramírez <[email protected]>

Author: Hussain-Tinwala

documentation/reference/sql/sample-by.md

@@ -50,6 +50,7 @@ Where the unit for sampled groups may be one of the following:
 | `m`  | minute      |
 | `h`  | hour        |
 | `d`  | day         |
+| `w`  | week        |
 | `M`  | month       |
 | `y`  | year        |